In high school I worked as an assistant for a professional photographer
and I learned that xenon strobes give off huge amounts of UV light.
This causes trouble is in wedding photography. It turns out that satin
wedding dresses fluoresce brightly under UV light! This results in a
bluish or purple cast to the dress and can really ruin the picture. To
avoid this, the Pros use a UV blocking filter over their strobes.
The second bit of info comes from an experiment I made to enhance the
coloration of some my Droseras with UV light. Since sunlight enhances
anthocyanin production much better than most artificial light, I thought
that this might be due to the extra UV in sunlight. To test this idea,
I took a short wavelength UV lamp from a EPROM eraser unit and directed
the light onto D. capensis plants. The results were devastating. A 10
second exposure from about 6" distance caused all the tentacles to dry
up and wither.
Could it be that Michael's strobe was unfiltered, but Barry's had a
plastic diffuser/filter built-in. A powerful strobe puts out about
50-100 watt-seconds of energy which is roughly comparable to my 10 watt
UV bulb for 10 seconds.
-- Rick